1. Field of the Invention
This invention relates to amplifiers, and, more particularly, to systems and methods for improving linearity of power amplifiers.
2. Description of the Related Art
Electronic amplifiers are used for increasing the power and/or amplitude of various specified signals. Most amplifiers operate by taking power from a power supply, and controlling an output signal to match the shape of the input signal, while providing a higher amplitude signal. One widely used type of electronic amplifier is the power amplifier (PA). Power amplifiers are versatile devices that are used in various applications to meet design requirements for signal conditioning, special transfer functions, analog instrumentation, and analog computation, among others. Power amplifiers are often used in wireless applications, and may employ radio frequency (RF) amplifier designs for use in the radio frequency range of the electromagnetic spectrum. An RF power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a signal of significant power, typically for driving the antenna of a transmitter. RF power amplifiers are oftentimes used to increase the range of a wireless communication system by increasing the output power of a transmitter.
Amplifiers are typically characterized based on their input and output characteristics. For instance, gain of an amplifier relates a magnitude of the amplifier's output signal to the magnitude of the amplifier's input signal. The gain may be specified as a ratio of an output voltage and an input voltage, or the ratio of an output power and an input power. Gain is often expressed as a transfer function of the amplifier that relates an input to an output. In most cases, the transfer function of an amplifier is expected to be linear, that is the gain is expected to have a linear relationship between the level of an input and the level of the resulting output. Nonlinear amplifiers may be subject to distortion that can vary the ratio of the input and output non-linearly, resulting in a distorted output that does not faithfully reflect the input being amplified. A non-linearity may manifest itself in an amplifier as an amplitude-dependent gain that decreases at higher amplitudes. For example, the effective gain of an amplifier may be lower for higher amplitude signals than for similar lower amplitude signals. This is commonly referred to as amplitude distortion. A non-linearity may also manifest itself as an amplitude-dependent phase shift. For example, the phase shift of the output signal may vary at different amplitudes. This is commonly referred to as phase distortion. Thus, although it is generally desirable that the output of amplifiers provide a faithful reproduction of the input signal, a faithful reproduction may not be provided due to inherent non-linearity of certain amplifiers. In an attempt to reduce the affects of the nonlinearities, the input signal may be kept low/small. Such a solution, however, may be undesirable for power amplifiers, as it limits the output power level, and lowers the efficiency of the amplifier.
Accordingly, it may be desirable to provide a technique that reduces nonlinearities across various output power levels for Power Amplifiers.